Rates of spontaneous mutation vary greatly among organisms. We have previously described genomic mutation rates that are characteristic of certain broad groups of organisms. For instance, riboviruses have genomic mutation rates of 0.1?1 per replication, a value approaching the maximum compatible with survival. The characteristic genomic mutation rate for DNA-based microbes is about 0.003 per replication based on eight very diverse organisms. In the Archaeon Sulfolobus acidocaldarius growing at 75 degrees C, however, the mutation rate is about 0.00075 and the proportion of base-pair substitutions is lower than in all other DNA-based organisms. We hypothesized that the average missense mutation has a greater deleterious effect at high temperatures. As a result, evolutionary pressures would act to decrease base-substitution mutation rates. One prediction of this hypothesis is that a thermophilic eubacterium would also show reduced frequencies of base-pair substitutions while a mesophilic archaeon would not, and such organisms are now being investigated. In a separate line of work, we are investigating mutants that have more mutations than expected on a random distribution, and this area will soon be our major focus.